Boys Will Be Boys - But Why?

Testosterone levels in male babies determined more by environment than by genetics

/ Author:  / Reviewed by: Chris Galloway, M.D.

(RxWiki News) If boys will be boys, that's generally because of the testosterone coursing through their bodies, but how much of this testosterone results from the environment and how much is genetic?

Although studies on teen and adults have found that heritability plays a big role in determining males' testosterone levels, a new study finds the opposite is true for baby boys.

Levels of testosterone in male infants appear to have more to do with their environment than their heritability.

"Follow your OB/GYN's or midwife's advice during prenatal appointments."

Senior author Richard Tremblay and lead author Doretta Caramaschi, both of the Research Unit on Children's Psychosocial Maladjustment at the University of Montreal in Quebec, Canada, joined colleagues to study 314 pairs of identical and fraternal twins.

They took saliva samples from each pair at 5 months old to measure the babies' levels of testosterone. Then they compared these levels between identical and fraternal (non-identical) twins to see how much influence on testosterone might be genetic versus how much might be environmental.

They found that the variation among twins that could be explained by common environmental factors was 56.6 percent, while 43.4 percent of the variation occurred alongside unique environmental factors.

In other words, environment is slightly edging out genes in determining testosterone levels in these twins.

The study was too limited to be able to determine what environmental conditions might be responsible, and to what extent, for a baby's testosterone levels.

Among those that could play a part are the mother's diet, whether the child's mother smokes, whether she breastfeeds her child and general interactions between the mother and her child.

“Because our study suggests that testosterone levels in infants are determined by the circumstances in which the child develops before and after birth, further studies will be needed to find out exactly what these influencing factors are and to what extent they change from birth to puberty,” Tremblay said.

Teasing out how much is environment and how much is genetic can help scientists understand how boys develop the way they do since testosterone is essential for normal sexual development.

It's also responsible for "typical boy behavior," such as social dominance and sexual behavior.

The study was published online May 7 in the journal Psychoneuroendocrinology. The research was funded by the Canadian Institutes of Health Research, the Social Sciences and Humanities Research Council of Canada, the Fonds de la recherche du Québec.

Reviewed by: 
Review Date: 
May 14, 2012
Last Updated:
July 9, 2012